1. Field of the Invention
The present invention relates to a three-phase alternating current electric motor which is provided with a structure which reduces torque ripple, more particularly relates to a distributed winding three-phase alternating current electric motor where the different phases of windings are wound distributed in several slots.
2. Description of the Related Art
Electric motors which are provided with permanent magnets suffer from pulsation called “cogging torque” and torque fluctuations called “torque ripple”. Cogging torque is a phenomenon where the magnetic attraction force between the armature and rotor in the electric motor finely vibrates depending on the rotation angle and can be confirmed when making the shaft of the electric motor rotate by hand etc. without running a current to the electric motor. The cogging torque changes in magnitude due to the shape of the rotor or stator etc. On the other hand, torque ripple occurs due to the waveform of the induced electromotive force in the electric motor deviating from the ideal waveform. The harmonics of the magnetic flux which is generated from the rotor (or the harmonics of the induced voltage which is generated from that magnetic flux) are the cause. The phenomenon is dependent on the magnitude of the current (torque ripple ∝ harmonics of induced voltage×current). In an electric motor, in particular, there is a tendency for the pulse components, comprising number of pole pairs×6 times vibration for one turn of the rotor, to be large.
As explained above, the torque which is generated from the electric motor has the above-mentioned cogging torque and torque ripple added to it as pulsation. Due to the presence of such pulsation, vibration and noise occur at the apparatus which is driven by the electric motor. In terms of magnitude of pulsation, when the load of the electric motor is small, the cogging torque is dominant, while when the load of the electric motor is large, the torque ripple is dominant. This is because when the load of the electric motor is small, the current for driving the electric motor becomes small, while when the load of the electric motor is large, the current for driving the electric motor becomes large.
The conventional method for reducing torque ripple was to optimize the shape of the core of the rotor or the shape of the core of the stator, the skew by which the magnetic pole boundaries of the core of the rotor were skewed from the axial direction, the skew at the core of the stator etc. However, remodification of an electric motor for optimization requires a more complicated structure of the electric motor and becomes a cause of increasing the manhours in production. Further, torque ripple is dependent on the magnitude of the current, so even if taking measures such as skew, the effect is small when using a relatively large current for a drive operation.
In this regard, in an electric motor, the number of magnetic poles (number of poles), the number of slots which hold the windings, and the layout of the windings in the slots also affect the magnitudes of the cogging torque and the torque ripple. As a combination of the number of poles and number of slots enabling the cogging torque and the torque ripple to be reduced, there is the “fractional slot” configuration where the number of slots divided by the number of poles becomes an irreducible fraction. There are electric motors which employ this fractional slot configuration (for example, see Japanese Patent Publication No. 2004-23950A and Japanese Patent Publication No. 7-106046B2).
In a fractional slot type electric motor, it is possible to select the number of poles and the number of slots so as to increase the least common multiple of the number of poles and the number of slots and possible to reduce the cogging torque and the torque ripple. However, among the harmonic components which are contained in the induced voltage of an electric motor, the relatively lower order ones such as the fifth and seventh harmonics do not completely disappear. The torque ripple, which occurs due to the harmonics of the induced voltage and the current, cannot be sufficiently reduced.
Further, in an electric motor in which the number of slots becomes three times or more the number of pole pairs and where the slots become fractional slots, while the cogging torque and the torque ripple tend to become smaller, the coil pitch of the windings which are inserted into the slots become larger than 1 slot and winding is only possible by distributed winding. In particular, in an electric motor where the value of the number of slots divided by the number of pole pairs, the number of phases, and 2 becomes an irreducible fraction and the value of the denominator becomes 4 or more, the layout of the windings becomes complicated, so lap winding is the general practice. This is not suited for automation of winding at the time of production.